Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752718Ab1FGEkU (ORCPT ); Tue, 7 Jun 2011 00:40:20 -0400 Received: from e6.ny.us.ibm.com ([32.97.182.146]:48339 "EHLO e6.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751190Ab1FGEkT (ORCPT ); Tue, 7 Jun 2011 00:40:19 -0400 Date: Mon, 6 Jun 2011 21:40:05 -0700 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: Milton Miller , Ingo Molnar , Peter Zijlstra , Thomas Gleixner , LKML Subject: Re: [1/4] rcu: Detect uses of rcu read side in extended quiescent states Message-ID: <20110607044005.GB2292@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1307329858-14999-3-git-send-email-fweisbec@gmail.com> <20110606181021.GL3066@linux.vnet.ibm.com> <20110607001905.GE17026@somewhere.redhat.com> <20110607004250.GZ3066@linux.vnet.ibm.com> <20110607013630.GF17026@somewhere.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20110607013630.GF17026@somewhere.redhat.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4970 Lines: 113 On Tue, Jun 07, 2011 at 03:36:32AM +0200, Frederic Weisbecker wrote: > On Mon, Jun 06, 2011 at 05:42:50PM -0700, Paul E. McKenney wrote: > > On Tue, Jun 07, 2011 at 02:19:07AM +0200, Frederic Weisbecker wrote: > > > On Mon, Jun 06, 2011 at 11:10:21AM -0700, Paul E. McKenney wrote: > > > > commit c15d76f26712bd5228aa0c6af7a7e7c492a812c9 > > > > Author: Paul E. McKenney > > > > Date: Tue May 24 08:31:09 2011 -0700 > > > > > > > > rcu: Restore checks for blocking in RCU read-side critical sections > > > > > > > > Long ago, using TREE_RCU with PREEMPT would result in "scheduling > > > > while atomic" diagnostics if you blocked in an RCU read-side critical > > > > section. However, PREEMPT now implies TREE_PREEMPT_RCU, which defeats > > > > this diagnostic. This commit therefore adds a replacement diagnostic > > > > based on PROVE_RCU. > > > > > > > > Because rcu_lockdep_assert() and lockdep_rcu_dereference() are now being > > > > used for things that have nothing to do with rcu_dereference(), rename > > > > lockdep_rcu_dereference() to lockdep_rcu_suspicious() and add a third > > > > argument that is a string indicating what is suspicious. This third > > > > argument is passed in from a new third argument to rcu_lockdep_assert(). > > > > Update all calls to rcu_lockdep_assert() to add an informative third > > > > argument. > > > > > > > > Finally, add a pair of rcu_lockdep_assert() calls from within > > > > rcu_note_context_switch(), one complaining if a context switch occurs > > > > in an RCU-bh read-side critical section and another complaining if a > > > > context switch occurs in an RCU-sched read-side critical section. > > > > These are present only if the PROVE_RCU kernel parameter is enabled. > > > > > > > > Again, you must enable PROVE_RCU to see these new diagnostics. But you > > > > are enabling PROVE_RCU to check out new RCU uses in any case, aren't you? > > > > > > > > Signed-off-by: Paul E. McKenney > > > > > > A little comment about this patch: > > > > > > > > > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > > > > index 88547c8..8b4b3da 100644 > > > > --- a/kernel/rcutree.c > > > > +++ b/kernel/rcutree.c > > > > @@ -153,6 +153,12 @@ void rcu_bh_qs(int cpu) > > > > */ > > > > void rcu_note_context_switch(int cpu) > > > > { > > > > + rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map), > > > > + "Illegal context switch in RCU-bh" > > > > + " read-side critical section"); > > > > + rcu_lockdep_assert(!lock_is_held(&rcu_sched_lock_map), > > > > + "Illegal context switch in RCU-sched" > > > > + " read-side critical section"); > > > > > > This looks like more a check to make inside might_sleep(). > > > It's better because might_sleep() triggers the check even if > > > we don't actually go to sleep. > > > > This does make quite a bit of sense. > > > > > In fact I believe might_sleep() already does the job fine: > > > > > > If !PREEMPT, might_sleep() detects that preemption is disabled > > > by rcu_read_lock(). > > > > If !PREEMPT, isn't the preempt_disable() called by rcu_read_lock() > > implemented as follows? > > > > #define preempt_disable() do { } while (0) > > > > Unless I am missing something, __might_sleep() won't detect that. > > Ah, right. > > > > If PREEMPT, might_sleep() checks rcu_preempt_depth(). > > > > Agreed, for CONFIG_TREE_PREEMPT_RCU and CONFIG_TINY_PREEMPT_RCU, > > the existing might_sleep() checks do cover it. > > > > So I could export an rcu_might_sleep() or some such that contained > > the above two rcu_lockdep_assert()s, but only if !PREEMPT_RCU. > > If PREEMPT_RCU, rcu_might_sleep() would be a no-op. > > > > Seem reasonable, or am I missing something? > > Ok but that only improves the rcu debugging. What about instead improving > might_sleep() to also work in !PREEMPT, so that it profits to any detection > of forbidden sleeping (sleep inside spinlock, preempt_disable, might_fault, etc...) > > We could define a new config: > > config PREEMPT_COUNT > default PREEMPT || DEBUG_SPINLOCK_SLEEP > > and build preempt_disable()/preempt_enable() on top of that instead > of using CONFIG_PREEMPT directly. > > Does that look sane? The bit I am missing is how to distinguish between spinlocks (where sleeping is illegal) and mutexes (where sleeping is perfectly fine). We could teach lockdep the difference, I suppose, but it is not clear to me that it is worth it. In contrast, with RCU, this is straightforward -- check for rcu_sched and rcu_bh, but not SRCU. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/